The superconducting properties of certain defect oxide type materials which were variations of a well-known class of inorganic structures called perovskites were first observed in 1986 by Bednorz and Mueller. The materials they observed contained lanthanum, barium, copper, and oxygen, and were reported to be superconducting at 30 degrees Kelvin. Subsequently, many workers in the field became involved in efforts that resulted in the increase of the transition temperature, T.sub.c, by the substitution of yttrium for lanthanum.
Workers in the field identified, separated, and characterized the superconducting composition as a perovskite defect oxide of the Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.y type, possibly an orthorhombically distorted perovskite (See, for example, R. J. Cava, B. Batlogg, R. B. van Dover, D. W. Murphy, S. Sunshine, T. Siegrist, J. P. Remeika, E. A. Reitman, S. Zahurak, and G. P. Espinosa, Bulk Superconductivity at 91K in Single-Phase Oxygen-Deficient Perovskite Ba.sub.2 YCu.sub.3 O.sub.9-delta' , Phys.Rev.Lett., Vol. 58, (16), pp. 1676-1679, Apr. 20, 1987, incorporated herein by reference, for the crystallographic parameters and the material parameters), also referred to as a tetragonal, square-planar structure (See P. H. Hor, R. L. Meng, Y. Q. Wang, L. Gao, Z. J. Huang, J. Bechtold, K. Forster, and C. W. Chu, Superconductivity Above 90K in The Square Planar Compound System ABa.sub.2 Cu.sub.3).sub.6+x With A=La, Nd, Sm, Gd, Er, and Lu, Mar. 17, 1987) with the Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.y, defect oxide perovskite phase being responsible for the superconducting properties. Further work with this phase effectively raised the critical temperature, T.sub.c, to a few degrees above 90 degrees Kelvin (a temperature above the atmospheric boiling point of liquid nitrogen).
Later workers in the field have attempted the total and/or partial replacement of the yttrium and/or lanthanum with other Group IIIA metals (including Rare Earths), especially with scandium, europium, lutetium, neodymium, praseodymium, gadolinium, and ytterbium. The same and other workers in the field have also attempted the total and/or partial replacement of barium with other Group IIA metals, as strontium and calcium.
The defect oxide perovskite phase, having the general composition M.sub.1.sup.IIIA M.sub.2.sup.IIA M.sub.3.sup.IB O.sub.y, was identified by several groups utilizing microprobe analysis, x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. These groups have independentally characterized this defect oxide perovskite, M.sub.1.sup.IIIA M.sub.2.sup.IIA M.sub.3.sup.IB O.sub.y phase as having the crystallographic unit cell structure shown in FIG. 1.
The perovskite structure is similar to the naturally occurring calcium titanate structure, CaTiO.sub.3, also shown by other AB.sub.2 O3-type oxides having at least one cation much larger then the other cation or cations, including the tungsten bronzes, NaWO.sub.3, strontium titanate, barium titanate, YAlO.sub.3, LaGa.sub.3, NaNbO.sub.3, KNbO.sub.3, KMgF.sub.3, KNiF.sub.3, and KZnF.sub.3, among others. In the perovskite structure the larger ions (La.sup.+3 =1.15 .ANG., Ba.sup.+2 =1.35 .ANG., and O.sup.+2 =1.40 .ANG., Linus Pauling, The Nature of the Chemical Bond, 3rd Edition, Table 13-3, "Crystal Radii and Univalent Radii of Ions") form a cubic close packed structure, with the smaller ions (Cu.sup.+2 =0.96 .ANG., Y.sup.+3 =0.90 .ANG., Pauling, op. cit.) arranged in occupying octahedral interstices in an ordered pattern. Together they form a cubic close packed (face centered cubic) array.
The superconducting perovskite type materials are defect oxides. That is, they are solids in which different kinds of atoms occupy structurally equivalent sites, where, in order to preserve electrical neutrality, some sites are unoccupied.
The structure shown in FIG. 1 has a recurring structure of (1) a M.sup.IB -O plane of a first type with vacant O sites, (2) a M.sup.IIA -O plane, (3) a M.sup.IB -O plane of a second type with fully occupied O sites, (4) a M.sup.IIIA plane with O sites, (5), another M.sup.IB -O plane of the first type with fully occupied O sites, (6) another plane of the M.sup.IIA -O type, and a (7) a second M.sup.IB -O plane of the first type, with O site vacancies. It may thus be seen that the unit cell so formed has seven planes.
The central plane is a plane of the M.sup.IIIA -O type, as a Y-O or La-O plane, with the Group IIIA metal being surrounded at its four coplanar corners by oxygen sites, which may be occupied or vacant. Immediately above and below this M.sup.IIIA -O plane are equivalent M.sup.IB -O planes of the second type, i.e., Cu-O planes, with the Group IB metal ions being at the four corners of the plane, and occupied oxygen sites being along each edge of the planes. These square planar M.sup.IB atoms (or ions), each surrounded by four oxygen atoms (or ions) have been reported to be critical to superconductivity in the defect oxide perovskites. A pair of M.sup.IIA -O planes, as Ba-O planes lie atop and below these fully occupied first type M.sup.IB -O planes. The M.sup.IIA -O planes, formed with the Group IIA metal, as barium, at the center have fully occupied oxygen sites, with the oxygens disposed directly above and below the Group IB metal sites of the adjacent planes. The M.sup.IIA -O planes are disposed between M.sup.IB -O planes, as shown in FIG. 1, with the first type M.sup.IB -O planes disposed on opposite sides thereof relative to the second type M.sup.IB -O planes. As mentioned above, the deficiencies, that is, the vacancies (unoccupied sites) reported to reside in the first type M.sup.IB -O planes are the result of the requirement of electrical neutrality. While the vacancies are generally reported to be in the M.sup.IB -O planes, they may also be in the other planes, as in the M.sup.IIA -O planes and/or in the M.sup.IIIA -O planes.
An accepted theory of superconductivity for "liquid helium temperature range superconducting materials described in the prior art is the Bardeen- Cooper- Schrieffer (BCS) Theory. The BCS Theory explains that superconductivity results from a pairing of conduction electrons. In the paired states electrons cannot easily be scattered and thus lose energy due to collisions with impurities and lattice vibrations. This is much the same loss mechanism that occurs for the unpaired electrons in normally conducting metals. It is this inability of the paired electrons to scatter that results in the zero electrical resistance of the superconductor. The BCS theory predicts the pairing is caused by a net attractive interaction between electrons as a result of interactions between the electrons and vibrations in the crystal lattice. At temperatures below T.sub.c the attractive interaction relative to thermally produced lattice vibrations is strong enough to produce electron pairing.
An equation which is a direct consequence of BCS theory describes the critical temperature material parameters in terms of the following relationship: EQU T.sub.c =1.14(.THETA..sub.D)EXP(-1/UD)
where T.sub.c is the critical temperature, (.THETA..sub.D) is the Debye Temperature, U is the electron-lattice phonon interaction constant, and D is the electron density of orbitals at the Fermi surface. The Debye temperature, (.THETA..sub.D) is given by the formula: ##EQU1## where h is Planck's Constant, v is the velocity of sound in the lattice, K.sub.B is Boltzmann's constant, and N is the number of atoms per volume V in the lattice.
Thus, according to BCS Theory, high T.sub.c superconductive materials are those materials with strong electron-phonon interactions, high Debye temperatures, and large electronic density of states at the Fermi surface. The electron-phonon interaction strength V*, multiplied by the carrier concentration (density of electrons) must be large for high values of T.sub.c. This is the case in oxides of nickel ions and of copper ions, which form Jahn-Teller polarons.
While the BCS Theory did not, itself, set an upper temperature limit for superconductivity, it was widely believed that real materials could not be formed having properties that would allow relatively high temperature (above 40-50 K.) superconductivity, given the range of material properties encountered in the materials investigated. However, the recent report of the development of the high temperature superconducting properties of the defect oxide perovskite type superconductors has shattered this once widely held belief.
High critical temperature of about 90 K. has now been reported for the perovskite type, defect oxide superconductors. This represents a much higher critical temperature, T.sub.c then would be predicted by the straight forward application of the preceeding equation from BCS theory.
Clearly, new physical properties (for those perovskite type defect oxide superconductors) are operative not envisioned in the BCS theory as it has been heretofore applied to "liquid helium" temperature range materials.
While the aforementioned researchers have made great strides forward over the course of the last several months in developing this new class of materials, they have not as yet been able to stabilize those materials, nor have they successfully moved the critical temperature a significant amount above the atmospheric boiling point of liquid nitrogen.